A FlashMap provides a way for one request to store attributes intended for
use in another. This is most commonly needed when redirecting from one URL
to another -- e.g. the Post/Redirect/Get pattern. A FlashMap is saved before
the redirect (typically in the session) and is made available after the
redirect and removed immediately.

A FlashMap can be set up with a request path and request parameters to
help identify the target request. Without this information, a FlashMap is
made available to the next request, which may or may not be the intended
recipient. On a redirect, the target URL is known and a FlashMap can be
updated with that information. This is done automatically when the
org.springframework.web.servlet.view.RedirectView is used.

Note: annotated controllers will usually not use FlashMap directly.
See org.springframework.web.servlet.mvc.support.RedirectAttributes
for an overview of using flash attributes in annotated controllers.